Oscillator, a clock generator and a method for generating a clock signal

ABSTRACT

An oscillator configured to generate an oscillation signal is provided. The oscillator includes a transistor pair and a cross-coupled transistor pair. The transistor pair is coupled to a first current source and has a first transconductance. The first transconductance is changed in response to a current value of the first current source. The cross-coupled transistor pair is coupled to a second current source and has a second transconductance. The second transconductance is changed in response to a current value of second current source. The transistor pair and the cross-coupled transistor pair are mutually coupled by a plurality of inductors. A frequency of the oscillation signal is determined according to the first transconductance and the second transconductance. Furthermore, a clock generator and a method for generating a clock signal thereof are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefits of U.S. provisionalapplication Ser. No. 62/074,091, filed on Nov. 3, 2014 and Taiwanapplication serial no. 104122492, filed on Jul. 13, 2015. The entiretyof each of the above-mentioned patent applications is herebyincorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electronic circuit element, a signalgenerator and a method for generating a signal, and particularly relatesto an oscillator, a clock generator and a method for generating a clocksignal.

2. Description of Related Art

An oscillator is an important component in many electronic systems, andmay be applied in varies electronic circuit devices. It is veryimportant for an oscillator to output an oscillation signal withaccurate and stable frequency. Regarding a current oscillator applied toan electronic circuit, common factors that may affect the oscillationfrequency include temperature variation, process variation andelectromagnetic interference, etc., and such factors may cause afrequency drift to influence accuracy and stableness of the frequency.According to the conventional technique, a method for resolving theabove problem is to adjust a capacitance of a varactor in the oscillatorto compensate the drifted oscillation frequency, so as to maintain theaccuracy and stableness of the oscillation frequency. However, thevaractor is liable to produce a non-ideal second order effect or producea parasitic capacitance in a high frequency circuit, and such problemmay influence an operation of the circuit component and reliabilitythereof.

SUMMARY OF THE INVENTION

The invention is directed to an oscillator, in which a cross-coupledtransistor pair and another transistor pair in the oscillator aremutually coupled through a plurality of inductors, so as to generate anoscillation signal.

The invention is directed to a clock generator including theaforementioned oscillator, which is configured to generate a clocksignal.

The invention is directed to a method for generating a clock signal,which is configured to control the aforementioned oscillator to generatean oscillation signal to serve as the clock signal.

The invention provides an oscillator configured to generate anoscillation signal. The oscillator includes a transistor pair and across-coupled transistor pair. The transistor pair is coupled to a firstcurrent source and has a first transconductance. The firsttransconductance is changed in response to a current value of the firstcurrent source. The cross-coupled transistor pair is coupled to a secondcurrent source and has a second transconductance. The secondtransconductance is changed in response to a current value of the secondcurrent source. The transistor pair and the cross-coupled transistorpair are mutually coupled through a plurality of inductors. A frequencyof the oscillation signal is determined according to the firsttransconductance and the second transconductance.

In an embodiment of the invention, the oscillator is a crystal-freeoscillator.

In an embodiment of the invention, the oscillator is avoltage-controlled oscillator. The voltage-controlled oscillator isconfigured to generate the oscillation signal according to an inputvoltage. The current value of at least one of the first current sourceand the second current source is adjusted according to the inputvoltage.

In an embodiment of the invention, the inductors include a firstinductor and a second inductor. The transistor pair includes a firsttransistor and a second transistor. The first transistor has a firstterminal, a second terminal and a control terminal. The first terminalis coupled to the first inductor. The second terminal is coupled to thefirst current source. The control terminal is coupled to thecross-coupled transistor pair. The second transistor has a firstterminal, a second terminal and a control terminal. The first terminalis coupled to the second inductor. The second terminal is coupled to thefirst current source. The control terminal is coupled to thecross-coupled transistor pair. At least one of the first terminal of thefirst transistor and the first terminal of the second transistor servesas an output terminal. The oscillator outputs the oscillation signalthrough the output terminal.

In an embodiment of the invention, the inductors further include a thirdinductor and a fourth inductor. The cross-coupled transistor pairincludes a third transistor and a fourth transistor. The thirdtransistor has a first terminal, a second terminal and a controlterminal. The first terminal is coupled to the third inductor and thecontrol terminal of the first transistor, and the second terminal iscoupled to the second current source. The fourth transistor has a firstterminal, a second terminal and a control terminal. The first terminalis coupled to the fourth inductor and the control terminal of the secondtransistor, and the second terminal is coupled to the second currentsource. The control terminal of the third transistor is coupled to thefirst terminal of the fourth transistor, and the control terminal of thefourth transistor is coupled to the first terminal of the thirdtransistor.

In an embodiment of the invention, the first inductor and the thirdinductor form a first mutual inductor. The second inductor and thefourth inductor form a second mutual inductor. The first mutual inductorand the second mutual inductor are physically isolated.

In an embodiment of the invention, the current value of at least one ofthe first current source and the second current source is adjustedaccording to a temperature parameter.

In an embodiment of the invention, the oscillator further includes atemperature sensor circuit. The temperature sensor circuit is coupled toat least one of the first current source and the second current source.The temperature sensor circuit is configured to sense the temperatureparameter, and adjusts the current value of at least one of the firstcurrent source and the second current source according to thetemperature parameter.

In an embodiment of the invention, the temperature sensor circuitadjusts the current value of at least one of the first current sourceand the second current source by using at least one of a third currentsource and a fourth current source.

In an embodiment of the invention, the third current source is selectedfrom one of a current source proportional to absolute temperature (PTAT)and a current source complementary to absolute temperature (CTAT). Thefourth current source is selected from another one of the current sourceproportional to absolute temperature and the current sourcecomplementary to absolute temperature.

In an embodiment of the invention, the current value of at least one ofthe first current source and the second current source is adjustedaccording to a process parameter.

In an embodiment of the invention, the oscillator further includes acompensation circuit. The compensation circuit is coupled to at leastone of the first current source and the second current source. Thecompensation circuit is configured to receive a compensation signal, andoutputs a compensation current according to the compensation signal, soas to adjust the current value of at least one of the first currentsource and the second current source.

The invention provides a clock generator configured to generate a clocksignal. The clock generator includes an oscillator. The oscillator isconfigured to generate an oscillation signal to serve as the clocksignal. The oscillator includes a transistor pair and a cross-coupledtransistor pair. The transistor pair is coupled to a first currentsource and has a first transconductance. The first transconductance ischanged in response to a current value of the first current source. Thecross-coupled transistor pair is coupled to a second current source andhas a second transconductance. The second transconductance is changed inresponse to a current value of the second current source. The transistorpair and the cross-coupled transistor pair are mutually coupled througha plurality of inductors. A frequency of the oscillation signal isdetermined according to the first transconductance and the secondtransconductance.

The invention provides a method for generating a clock signal, which isconfigured to control an oscillator to generate an oscillation signal toserve as a clock signal. The oscillator includes a transistor pair and across-coupled transistor pair. The method for generating the clocksignal includes following steps. A first transconductance of thetransistor pair is determined according to a first current source, and asecond transconductance of the cross-coupled transistor pair isdetermined according to a second current source. A frequency of theoscillation signal is determined according to the first transconductanceand the second transconductance. The transistor pair and thecross-coupled transistor pair are mutually coupled through a pluralityof inductors. The transistor pair is coupled to the first currentsource. The first transconductance is changed in response to a currentvalue of the first current source. The cross-coupled transistor pair iscoupled to the second current source. The second transconductance ischanged in response to a current value of the second current source.

In an embodiment of the invention, the oscillator is a crystal-freeoscillator.

In an embodiment of the invention, the oscillator is avoltage-controlled oscillator configured to generate the oscillationsignal according to an input voltage. The step of determining the firsttransconductance of the transistor pair according to the first currentsource, and determining the second transconductance of the cross-coupledtransistor pair according to the second current source includesadjusting the current value of at least one of the first current sourceand the second current source according to the input voltage, so as tocorrespondingly change at least one of the first transconductance of thetransistor pair and the second transconductance of the cross-coupledtransistor pair.

In an embodiment of the invention, the step of adjusting the currentvalue of at least one of the first current source and the second currentsource according to the input voltage, so as to correspondingly changeat least one of the first transconductance of the transistor pair andthe second transconductance of the cross-coupled transistor pairincludes at least one of two following steps. The current value of thefirst current source is adjusted to change the first transconductance ofthe transistor pair. The current value of the second current source isadjusted to change the second transconductance of the cross-coupledtransistor pair.

In an embodiment of the invention, the inductors include a firstinductor, a second inductor, a third inductor and a fourth inductor. Thefirst inductor and the third inductor form a first mutual inductor. Thesecond inductor and the fourth inductor form a second mutual inductor.The first mutual inductor and the second mutual inductor are physicallyisolated.

In an embodiment of the invention, the method for generating the clocksignal further includes generating a temperature parameter, and using atleast one of a third current source and a fourth current source toadjust the current value of at least one of the first current source andthe second current source according to the temperature parameter.

In an embodiment of the invention, the third current source is selectedfrom one of a current source proportional to absolute temperature (PTAT)and a current source complementary to absolute temperature (CTAT). Thefourth current source is selected from another one of the current sourceproportional to absolute temperature (PTAT) and the current sourcecomplementary to absolute temperature (CTAT).

In an embodiment of the invention, the method for generating the clocksignal further includes receiving a compensation signal, and outputtinga compensation current according to the compensation signal, so as toadjust the current value of at least one of the first current source andthe second current source.

According to the above descriptions, in the exemplary embodiments of theinvention, the oscillator is configured to output an oscillation signal,which is used in the clock generator to serve as a clock signal. Thecross-coupled transistor pair and the transistor pair are mutuallycoupled through a plurality of inductors. The frequency of theoscillation signal is determined by the first transconductance and thesecond transconductance, so as to generate the oscillation signal.

In order to make the aforementioned and other features and advantages ofthe invention comprehensible, several exemplary embodiments accompaniedwith figures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a circuit schematic diagram of an oscillator according to anembodiment of the invention.

FIG. 2 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention.

FIG. 3 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention.

FIG. 4 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention.

FIG. 5 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention.

FIG. 6 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention.

FIG. 7 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention.

FIG. 8 is a circuit schematic diagram of a clock generator according toan embodiment of the invention.

FIG. 9 is a flowchart illustrating a method for generating a clocksignal according to an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

A plurality of embodiments are provided below to describe thedisclosure, though the disclosure is not limited to the providedembodiments, and the embodiments may also be suitably combined. A term“couple” used in the full text of the disclosure (including the claims)refers to any direct and indirect connections. For example, if a firstdevice is described to be coupled to a second device, it is interpretedas that the first device is directly coupled to the second device, orthe first device is indirectly coupled to the second device throughother devices or connection means. Moreover, a term “signal” refers toat least a current, a voltage, an electric charge, a temperature, dataor any other one or a plurality of signals.

FIG. 1 is a circuit schematic diagram of an oscillator according to anembodiment of the invention. Referring to FIG. 1, the oscillator 110 ofthe present embodiment includes a transistor pair M1, M3 and across-coupled transistor pair M2, M4, a first mutual inductor 112 and asecond mutual inductor 114. In the present embodiment, the transistorpair M1, M3 is coupled to a first current source I1. The cross-coupledtransistor pair M2, M4 is coupled to a second current source I2. Thefirst mutual inductor 112 includes a first inductor L1 and a thirdinductor L3, and the second mutual inductor 114 includes a secondinductor L2 and a fourth inductor L4. The first mutual inductor 112 andthe second mutual inductor 114 are physically isolated, and none mutualinductance value exists therebetween. In the present embodiment, thecross-coupled transistor pair M2, M4 and the transistor pair M1, M3 aremutually coupled through a plurality of the inductors L1 to L4.

To be specific, in the present embodiment, the transistor pair M1, M3includes a first transistor M1 and a second transistor M3. The firsttransistor M1 has a first terminal, a second terminal and a controlterminal. In the present embodiment, the first transistor M1 is, forexample, an n-channel metal-oxide-semiconductor field-effect transistor(NMOSFET), so that the first terminal, the second terminal and thecontrol terminal thereof are respectively a drain, a source and a gateof the NMOS transistor, though the invention is not limited thereto. Inthe present embodiment, the first terminal of the first transistor M1 iscoupled to the first inductor L1. The second terminal of the firsttransistor M1 is coupled to the first current source I1. The controlterminal of the first transistor M1 is coupled to the cross-coupledtransistor pair M2, M4 through a node X. The second transistor M3 has afirst terminal, a second terminal and a control terminal. Similar to thefirst transistor M1, in the exemplary embodiment of the NMOS transistor,the first terminal, the second terminal and the control terminal of thesecond transistor M3 are respectively a drain, a source and a gate ofthe NMOS transistor, though the invention is not limited thereto. Thefirst terminal of the second transistor M3 is coupled to the secondinductor L2. The second terminal of the second transistor M3 is coupledto the first current source I1. The control terminal of the secondtransistor M3 is coupled to the cross-coupled transistor pair M2, M4through a node Y. In the present embodiment, the first terminal of thefirst transistor M1 and the first transistor of the second transistor M3respectively serve as output terminals Vo+, Vo− of the oscillator 110.The oscillator 110 outputs an oscillation signal Vosc through the outputterminals Vo+, Vo−, as shown in FIG. 8. In the present embodiment, theoscillation signal Vosc is, for example, a differential signal, so thatthe first terminal of the first transistor M1 and the first transistorof the second transistor M3 respectively serve as the output terminalsVo+, Vo− of the oscillator 110. In an exemplary embodiment, theoscillator 110 may be a single-end output, which is not limited by theinvention. In the exemplary embodiment of the single-end output, thefirst terminal of the first transistor M1 and the first terminal of thesecond transistor M3 are taken as the output terminal of the oscillator110, which is determined according to an actual circuit design.

On the other hand, the cross-coupled transistor pair M2, M4 includes athird transistor M2 and a fourth transistor M4. The third transistor M2has a first terminal, a second terminal and a control terminal. Similarto the first transistor M1, in the exemplary embodiment of the NMOStransistor, the first terminal, the second terminal and the controlterminal of the third transistor M2 are respectively a drain, a sourceand a gate of the NMOS transistor, though the invention is not limitedthereto. The first terminal of the third transistor M2 is coupled to thethird inductor L3 and the control terminal of the first transistor M1through the node X, and the second terminal of the third transistor M2is coupled to the second current source I2. The fourth transistor M4 hasa first terminal, a second terminal and a control terminal. Similar tothe first transistor M1, in the exemplary embodiment of the NMOStransistor, the first terminal, the second terminal and the controlterminal of the fourth transistor M4 are respectively a drain, a sourceand a gate of the NMOS transistor, though the invention is not limitedthereto. The first terminal of the fourth transistor M4 is coupled tothe fourth inductor L4 and the control terminal of the second transistorM3 through the node Y. The second terminal of the fourth transistor M4is coupled to the second current source I2. In the present embodiment,in the cross-coupled transistor pair M2, M4, the control terminal of thethird transistor M2 is coupled to the first terminal of the fourthtransistor M4, and the control terminal of the fourth transistor M4 iscoupled to the first terminal of the third transistor M2.

In the present embodiment, although the transistors M1-M4 areimplemented by the NMOS transistors, the invention is not limitedthereto. In an embodiment, the transistors M1-M4 may also be implementedby p-channel metal-oxide-semiconductor field-effect transistor(PMOSFET). By using the PMOS transistors, a layout of the other circuitcomponents in the oscillator 110 may be adaptively adjusted, and sinceenough instructions and recommendations for the adjusting method thereofmay be learned according to general knowledge of related technicalfield, details thereof are not repeated. Moreover, according to anactual circuit design requirement, the oscillator 110 may include ordoes not include the first current source I1 and the second currentsource I2. Moreover, the first mutual inductor 112 and the second mutualinductor 114 and the oscillator 110 may be fabricated together in anembedded manner or fabricated separately in an external-coupling manner,which is not limited by the invention.

In the present embodiment, the transistor pair M1, M3 has a firsttransconductance gm1. The first transconductance gm1 is obtained bycalculating a small signal model of the transistor pair M1, M3. Thefirst transconductance gm1 is changed in response to a current value ofthe first current source I1. The cross-coupled transistor pair M2, M4has a second transconductance gm2. The second transconductance gm2 isobtained by calculating a small signal model of the cross-coupledtransistor pair M2, M4. The second transconductance gm2 is changed inresponse to a current value of the second current source I2. In thepresent embodiment, a frequency of the oscillation signal Vosc generatedby the oscillator 110 is determined according to the firsttransconductance gm1 and the second transconductance gm2.

To be specific, in the present embodiment, the frequency of theoscillation signal Vosc is, for example, determined by a followingequation:

$\omega_{osc}^{2} = \frac{C - {\left( {{g_{m\; 1}K} + g_{m\; 2}} \right)g_{m\; 2}L}}{{LC}^{\; 2}}$

Where, ωsc is the frequency of the oscillation signal Vosc, gm1 is thefirst transconductance, gm2 is the second transconductance, K is amutual inductance value of the first mutual inductor 112 or a mutualinductance value of the second mutual inductor 114, C is an equivalentcapacitance value of a parasitic capacitor of the oscillator 110, and Lis a sum of an equivalent inductance value of a parasitic inductor ofthe oscillator 110 and inductance values of the inductors L1-L4.

In the present embodiment, the cross-coupled transistor pair M2, M4 ismagnetically coupled to the transistor pair M1, M3 through the firstmutual inductor 112 and the second mutual inductor 114, and theinductance values of the inductors L1-L4 are not changed. Therefore, thefrequency of the oscillation signal Vosc generated by the oscillator 110may be determined by the first transconductance gm1 and the secondtransconductance gm2. Moreover, in the present embodiment, theoscillator 110 is, for example, a crystal-free oscillator.

In an embodiment of the invention, the current value of at least one ofthe first current source I1 and the second current source I2 may beadjusted according to an input voltage, so as to correspondingly adjustthe first transconductance gm1 and the second transconductance gm2.

FIG. 2 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention. Referring to FIG. 1 and FIG. 2, theoscillator 210 of the present embodiment is similar to the oscillator110 of FIG. 1, and a main difference therebetween is that the oscillator210 is a voltage-controlled oscillator (VCO), which is electricallyconnected to a voltage control circuit 220.

To be specific, in the present embodiment, the first current source I1and the second current source I2 are, for example, implemented bycurrent mirror circuits. For example, a combination of transistors M5,M7 and M9 is a first current mirror, which is configured to map a biascurrent source I5 to the oscillator 210 to serve as the first currentsource I1. A combination of transistors M6, M7 and M10 is a secondcurrent mirror, which is configured to map a bias current source I6 tothe oscillator 210 to serve as the second current source I2. In thepresent embodiment, the transistors M5, M6 serve as an input transistorpair for respectively receiving input voltages from input terminals Vin+and Vin−. A current Ic+ and a current Ic− respectively drained by thefirst current mirror and the second current mirror from the bias currentsource I5 and the bias current source I6 are changed along withdifferent input voltages of the input terminals Vin+ and Vin−, so as toadjust current values mapped to the oscillator 210.

Therefore, in the present embodiment, the voltage control circuit 220receives the input voltages of the input terminals Vin+ and Vin−, so asto adjust the current values of the first current source I1 and thesecond current source I2 to change the first transconductance gm1 andthe second transconductance gm2. Therefore, the oscillator 210 is, forexample, a voltage-controlled oscillator. Moreover, according to anactual circuit design requirement, the oscillator 210 may include ordoes not include the voltage control circuit 220, which is not limitedby the invention. In the present embodiment, although the voltagecontrol circuit 220 is implemented through a double-end input, theinvention is not limited thereto. In an embodiment, the voltagecontrolled circuit 220 may also be a single-end input. In other words,the circuit structure of the voltage control circuit 220 is not limitedby the invention. Moreover, since enough instructions andrecommendations for the method for determining the frequency of theoscillation signal generated by the oscillator 210 of the presentembodiment may be learned from the descriptions of the embodiment ofFIG. 1, detailed description thereof is not repeated.

In an embodiment of the invention, a current value of at least one ofthe first current source I1 and the second current source I2 may beadjusted according to a temperature parameter, so as to correspondinglyadjust the first transconductance gm1 or the second transconductancegm2.

FIG. 3 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention. Referring to FIG. 1 and FIG. 3, theoscillator 310 of the present embodiment is similar to the oscillator110 of FIG. 1, and a main difference therebetween is that the oscillator310 includes a temperature sensor circuit 320, so as to compensate anoscillation frequency of the oscillator 310 drifted due to a temperaturevariation.

To be specific, in the present embodiment, the first current source I1is, for example, implemented by a current mirror circuit. For example,the current mirror is, for example, a combination of transistors M7 andM9, and is configured to map at least one of a third current source I3and a fourth current source I4 to the oscillator 310 to serve as thefirst current source I1. In the present embodiment, the temperaturesensor circuit 320 is coupled to the first current source I1 implementedby the current mirror. The temperature sensor circuit 320 is configuredto sense a temperature parameter, and adjust a current value of thefirst current source I1 according to the temperature parameter. In thepresent embodiment, the third current source I3 and the fourth currentsource I4 are configured to provide bias currents of the temperaturesensor circuit 320. The temperature sensor circuit 320, for example,adjusts the current value of the first current source I1 by using atleast one of the third current source I3 and the fourth current sourceI4. For example, in the present embodiment, the third current source I3is, for example, a current source proportional to absolute temperature(PTAT). The fourth current source I4 is, for example, a current sourcecomplementary to absolute temperature (CTAT). However, the invention isnot limited thereto, and in an embodiment, the third current source I3may also be a current source complementary to absolute temperature, andthe fourth current source I4 may also be a current source proportionalto absolute temperature.

Therefore, in the present embodiment, the temperature sensor circuit 320is configured to sense the temperature parameter, and adjust the currentvalue of the first current source I1 according to the temperatureparameter, so as to change the first transconductance gm1. Therefore,the temperature sensor circuit 320 may compensate the oscillationfrequency of the oscillator 310 drifted due to the temperaturevariation. Moreover, according to an actual circuit design requirement,the oscillator 310 may include or does not include the temperaturesensor circuit 320, the third current source I3 and the fourth currentsource I4.

In the present embodiment, since enough instructions and recommendationsfor implementations of the temperature sensor circuit 320, the currentsource proportional to absolute temperature and the current sourcecomplementary to absolute temperature may be learned according togeneral knowledge of the related technical field, details thereof arenot repeated. Moreover, since enough instructions and recommendationsfor the method for determining the frequency of the oscillation signalgenerated by the oscillator 310 of the present embodiment may be learnedfrom the descriptions of the embodiment of FIG. 1, detailed descriptionthereof is not repeated.

In an embodiment, although the implementation that the temperaturesensor circuit 320 adjusts the current value of the first current sourceI1 according to the temperature parameter is taken as an example fordescription, the invention is not limited thereto. In an embodiment, thetemperature sensor circuit 320 may also respectively adjust the currentvalues of the first current source I1 and the second current source I2according to the temperature parameter.

FIG. 4 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention. Referring to FIG. 3 and FIG. 4, theoscillator 410 of the present embodiment is similar to the oscillator310 of FIG. 3, though a main difference therebetween is that thetemperature sensor circuit 420 respectively adjusts the current valuesof the first current source I1 and the second current source I2according to the temperature parameter.

To be specific, in the present embodiment, the first current source I1and the second current source I2 are, for example, respectivelyimplemented by a current mirror circuit. For example, a combination ofthe transistors M7 and M9 is a first current mirror, which is configuredto map one of the third current source I3 and the fourth current sourceI4 to the oscillator 410 to serve as the first current source I1. Acombination of the transistors M8 and M10 is a second current mirror,which is configured to map another one of the third current source I3and the fourth current source I4 to the oscillator 410 to serve as thesecond current source I2. In the present embodiment, the temperaturesensor circuit 320 is coupled to the first current source I1 implementedby the first current mirror and the second current source I2 implementedby the second current mirror. The temperature sensor circuit 320 isconfigured to sense a temperature parameter, and adjust the currentvalues of the first current source I1 and the second current source I2according to the temperature parameter.

Moreover, since enough instructions and recommendations for the methodfor determining the frequency of the oscillation signal generated by theoscillator 410 of the present embodiment may be learned from thedescriptions of the embodiment of FIG. 1, detailed description thereofis not repeated.

In other words, in an exemplary embodiment of the invention, the currentvalue of at least one of the first current source I1 and the secondcurrent source I2 may be adjusted according to the temperatureparameter, so as to change the first transconductance gm1 and the secondtransconductance gm2, and accordingly compensate the oscillationfrequency of the oscillator drifted due to the temperature variation.

In an embodiment of the invention, the current value of at least one ofthe first current source I1 and the second current source I2 may beadjusted according to a process parameter, so as to correspondinglyadjust the first transconductance gm1 and the second transconductancegm2.

FIG. 5 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention. Referring to FIG. 1 and FIG. 5, theoscillator 510 of the present embodiment is similar to the oscillator110 of FIG. 1, and a main difference therebetween is that the oscillator510 includes a compensation circuit 530 to compensate an oscillationfrequency of the oscillator 510 drifted due to a process variation.

To be specific, in the present embodiment, the first current source I1is, for example, implemented by a current mirror circuit. For example, acombination of the transistors M7 and M9 is the current mirror, which isconfigured to map a current provided by the compensation circuit 530 tothe oscillator 510 to serve as the first current source I1. In thepresent embodiment, the compensation circuit 530 is coupled to thecurrent mirror to implement the first current source I1. Thecompensation circuit 530 is configured to receive compensation signalsD0-Dn, and outputs a compensation current according to the compensationsignals D0-Dn, so as to adjust the current value of the first currentsource I1. In the present embodiment, the compensation circuit 530includes a plurality of combinations of switches and current sourcescoupled in series with each other. These combinations are set inparallel, and the number and coupling method thereof are not used forlimiting the invention. The switches are turned on/off in response tothe compensation signals D0-Dn, so as to conduct a current transmissionpath. The current sources corresponding to the turned-on switches mayprovide currents, and a sum of the currents serves as the compensationcurrent, which is output to the current mirror from the compensationcircuit 530, so as to adjust the current value of the first currentsource I1 according to the compensation signals D0-Dn. In the presentembodiment, the compensation signals D0-Dn are, for example, signals setaccording to the process variation, so as to compensate the driftedoscillation frequency.

Therefore, in the present embodiment, the compensation circuit 530 isconfigured to adjust the current value of the first current source I1according to a process parameter, so as to change the firsttransconductance gm1. Therefore, the compensation circuit 530 maycompensate the oscillation frequency of the oscillator 510 drifted dueto the process variation. Moreover, according to an actual circuitdesign requirement, the oscillator 510 may include or does not includethe compensation circuit 530, which is not limited by the invention.

In the present embodiment, since enough instructions and recommendationsfor implementation of the compensation circuit 530 may be learnedaccording to general knowledge of related technical field, detailsthereof are not repeated. Moreover, since enough instructions andrecommendations for the method for determining the frequency of theoscillation signal generated by the oscillator 510 of the presentembodiment may be learned from the descriptions of the embodiment ofFIG. 1, detailed description thereof is not repeated.

In the present embodiment, although the implementation that thecompensation circuit 530 adjusts the current value of the first currentsource I1 according to the process parameter is taken as an example fordescription, the invention is not limited thereto. In an embodiment, thecompensation circuit 530 may also adjust the current value of the secondcurrent source I2 according to the process parameter. In an embodiment,the compensation circuit 530 may also respectively adjust the currentvalues of the first current source I1 and the second current source I2according to the process parameter. In other words, the current value ofat least one of the first current source I1 and the second currentsource I2 may be adjusted according to the process parameter, so as tocorrespondingly adjust the first transconductance gm1 or the secondtransconductance gm2.

FIG. 6 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention. Referring to FIG. 1 and FIG. 6, theoscillator 610 of the present embodiment is similar to the oscillator110 of FIG. 1, and a main difference therebetween is that the oscillator610 includes a temperature sensor circuit 620 and a compensation circuit630, so as to compensate the oscillation frequency of the oscillator 610drifted due to a temperature variation and a process variation.

To be specific, in the present embodiment, the first current source I1is, for example, implemented by a current mirror circuit. For example, acombination of the transistors M7 and M9 is the current mirror, which isconfigured to map at least one of the third current source I3, thefourth current source I4, a current provided by the compensation circuit630 to the oscillator 610 to serve as the first current source I1. Inthe present embodiment, the temperature sensor circuit 620 and thecompensation circuit 630 are coupled to the first current source I1implemented by the current mirror. The temperature sensor circuit 620and the compensation circuit 630 operate in collaboration torespectively compensate the oscillation frequency of the oscillator 610drifted due to the temperature variation and the process variation, andsince enough instructions and recommendations for the compensationmethod thereof may be learned from the descriptions of the embodimentsof FIG. 3 and FIG. 5, detailed description thereof is not repeated.Moreover, since enough instructions and recommendations for the methodfor determining the frequency of the oscillation signal generated by theoscillator 610 of the present embodiment may be learned from thedescriptions of the embodiment of FIG. 1, detailed description thereofis not repeated.

In the present embodiment, although the implementation that thetemperature sensor circuit 620 and the compensation circuit 630 operatein collaboration to adjust the current value of the first current sourceI1 is taken as an example for description, the invention is not limitedthereto. In an embodiment, the temperature sensor circuit 620 and thecompensation circuit 630 may also operate in collaboration to adjust thecurrent value of the second current source I2. In an embodiment, thetemperature sensor circuit 620 and the compensation circuit 630 may alsooperate in collaboration to respectively adjust the current values ofthe first current source I1 and the second current source I2.

FIG. 7 is a circuit schematic diagram of an oscillator according toanother embodiment of the invention. Referring to FIG. 5 and FIG. 7, theoscillator 710 of the present embodiment is similar to the oscillator510 of FIG. 5, and a main difference therebetween is that the oscillator710 further includes a temperature sensor circuit 720, so as tocompensate an oscillation frequency of the oscillator 710 drifted due toa temperature variation. In the present embodiment, the temperaturesensor circuit 720 and the compensation circuit 730 are, for example,respectively configured to adjust the current values of the secondcurrent source I2 and the first current source I1.

To be specific, in the present embodiment, the second current source I2is, for example, implemented by a current mirror circuit. For example, acombination of the transistors M8 and M10 is another current mirror,which is configured to map at least one of the third current source I3and the fourth current source I4 to the oscillator 710 to serve as thesecond current source I2. In the present embodiment, the temperaturesensor circuit 720 is coupled to the second current source I2implemented by the another current mirror. The temperature sensorcircuit 720 is configured to sense a temperature parameter, and adjust acurrent value of the second current source I2 according to thetemperature parameter, and since enough instructions and recommendationsfor the adjusting method thereof may be learned from the descriptions ofthe embodiment of FIG. 3, detailed description thereof is not repeated.

Moreover, in the present embodiment, since enough instructions andrecommendations for the method that the compensation circuit 730 adjuststhe current value of the first current source I1 and the method fordetermining the frequency of the oscillation signal generated by theoscillator 710 of the present embodiment may be learned from thedescriptions of the embodiments of FIG. 1 and FIG. 5, detaileddescription thereof is not repeated.

FIG. 8 is a circuit schematic diagram of a clock generator according toan embodiment of the invention. Referring to FIG. 8, the clock generator800 of the present embodiment is configured to generate a clock signalCLK. The clock generator 800 includes an oscillator 810. The oscillator810 is configured to generate an oscillation signal Vosc to serve as theclock signal CLK. In the present embodiment, the oscillator 810 is, forexample, the oscillators shown in FIG. 1 to FIG. 7, which is not limitedby the invention. In the present exemplary embodiment, the clockgenerator 800 may further include functional components such as aphase/frequency detector, a charge pump, a loop filter, a controlcircuit and a feedback circuit, etc., which is not limited by theinvention.

Moreover, in the present embodiment, since enough instructions andrecommendations for the method that the oscillator 810 generates theoscillation signal and the method for determining the frequency of theoscillation signal generated by the oscillator 810 may be learned fromthe descriptions of the embodiments of FIG. 1 to FIG. 7, detaileddescription thereof is not repeated.

FIG. 9 is a flowchart illustrating a method for generating a clocksignal according to an embodiment of the invention. Referring to FIG. 1,FIG. 8 and FIG. 9, the method for generating the clock signal of thepresent embodiment may be applied to the clock generator 800 of FIG. 8for controlling the oscillator 810 to generate the oscillation signalVosc to serve as the clock signal CLK, though the invention is notlimited thereto.

To be specific, in step S900, the oscillator 810 determines the firsttransconductance gm1 of the transistor pair according to the firstcurrent source I1, and determines the second transconductance gm2 of thecross-coupled transistor pair according to the second current source I2.Then, in step S910, the oscillator 810 determines a frequency of theoscillation signal Vosc according to the first transconductance gm1 andthe second transconductance gm2. Then, in step S920, the oscillator 810generates the oscillation signal Vosc to serve as the clock signal CLKof the clock generator 800 by using the transistor pair M1, M3 and thecross-coupled transistor pair M2, M4. In the present embodiment, thecurrent value of at least one of the first current source I1 and thesecond current source I2 may be adjusted according to one of the inputvoltage, the temperature parameter and the process parameter, so as tocompensate the oscillation frequency drifted due to the temperaturevariation and the process variation.

Moreover, in the present embodiment, since enough instructions andrecommendations for the method for generating the clock signal may belearned from the descriptions of the embodiments of FIG. 1 to FIG. 8,detailed description thereof is not repeated.

In summary, in the exemplary embodiments of the invention, theoscillator is configured to output an oscillation signal, which is usedin the clock generator to serve as a clock signal. A frequency of theoscillation signal may be drifted due to an influence of factors such astemperature variation, process variation or electromagneticinterference, etc. In the exemplary embodiments of the invention, theinductors in the oscillator may have a mutual induction effect to couplethe transistor pair and the cross-coupled transistor pair, such that thetransconductance of at least one of the transistor pair and thecross-coupled transistor pair may be changed in response to the currentvalue of the current source coupled thereto. Then, in the exemplaryembodiment of the invention, by using the temperature sensor circuit tosense the temperature parameter or using the compensation circuit to setthe compensation current corresponding to the process parameter, thecurrent source coupled to at least one of the transistor pair and thecross-coupled transistor pair may be adjusted, so as to compensate theoscillation frequency drifted due to the temperature variation or theprocess variation.

It will be apparent to those skilled in the art that variousmodifications and variations may be made to the structure of theinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the invention covermodifications and variations of this invention provided they fall withinthe scope of the following claims and their equivalents.

What is claimed is:
 1. An oscillator, configured to generate an oscillation signal, and the oscillator comprising: a transistor pair, coupled to a first current source, wherein the transistor pair has a first transconductance, and the first transconductance is changed in response to a current value of the first current source; and a cross-coupled transistor pair, coupled to a second current source, wherein the cross-coupled transistor pair has a second transconductance, the second transconductance is changed in response to a current value of the second current source, and the transistor pair and the cross-coupled transistor pair are mutually coupled through a plurality of inductors, wherein a frequency of the oscillation signal is determined according to the first transconductance and the second transconductance.
 2. The oscillator as claimed in claim 1, wherein the oscillator is a crystal-free oscillator.
 3. The oscillator as claimed in claim 1, wherein the oscillator is a voltage-controlled oscillator configured to generate the oscillation signal according to an input voltage, wherein the current value of at least one of the first current source and the second current source is adjusted according to the input voltage.
 4. The oscillator as claimed in claim 1, wherein the inductors comprise a first inductor and a second inductor, and the transistor pair comprises: a first transistor, having a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the first inductor, the second terminal is coupled to the first current source, and the control terminal is coupled to the cross-coupled transistor pair; and a second transistor, having a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the second inductor, the second terminal is coupled to the first current source, and the control terminal is coupled to the cross-coupled transistor pair, wherein at least one of the first terminal of the first transistor and the first terminal of the second transistor serves as an output terminal, and the voltage-controlled oscillator outputs the oscillation signal through the output terminal.
 5. The oscillator as claimed in claim 4, wherein the inductors further comprise a third inductor and a fourth inductor, and the cross-coupled transistor pair comprises: a third transistor, having a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the third inductor and the control terminal of the first transistor, and the second terminal is coupled to the second current source; and a fourth transistor, having a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the fourth inductor and the control terminal of the second transistor, and the second terminal is coupled to the second current source, wherein the control terminal of the third transistor is coupled to the first terminal of the fourth transistor, and the control terminal of the fourth transistor is coupled to the first terminal of the third transistor.
 6. The oscillator as claimed in claim 5, wherein the first inductor and the third inductor form a first mutual inductor, the second inductor and the fourth inductor form a second mutual inductor, and the first mutual inductor and the second mutual inductor are physically isolated.
 7. The oscillator as claimed in claim 1, wherein the current value of at least one of the first current source and the second current source is adjusted according to a temperature parameter.
 8. The oscillator as claimed in claim 7, further comprising: a temperature sensor circuit, coupled to at least one of the first current source and the second current source, configured to sense the temperature parameter, and adjusting the current value of at least one of the first current source and the second current source according to the temperature parameter.
 9. The oscillator as claimed in claim 8, wherein the temperature sensor circuit adjusts the current value of at least one of the first current source and the second current source by using at least one of a third current source and a fourth current source.
 10. The oscillator as claimed in claim 9, wherein the third current source is selected from one of a current source proportional to absolute temperature (PTAT) and a current source complementary to absolute temperature (CTAT), and the fourth current source is selected from another one of the current source proportional to absolute temperature and the current source complementary to absolute temperature.
 11. The oscillator as claimed in claim 1, wherein the current value of at least one of the first current source and the second current source is adjusted according to a process parameter.
 12. The oscillator as claimed in claim 11, further comprising: a compensation circuit, coupled to at least one of the first current source and the second current source, configured to receive a compensation signal, and outputting a compensation current according to the compensation signal, so as to adjust the current value of at least one of the first current source and the second current source.
 13. A clock generator, configured to generate a clock signal, the clock generator comprising: the oscillator as claimed in claim 1, configured to generate the oscillation signal to serve as the clock signal.
 14. A method for generating a clock signal, configured to control an oscillator to generate an oscillation signal to serve as the clock signal, wherein the oscillator comprises a transistor pair and a cross-coupled transistor pair, the method for generating the clock signal comprises: determining a first transconductance of the transistor pair according to a first current source, and determining a second transconductance of the cross-coupled transistor pair according to a second current source; and determining a frequency of the oscillation signal according to the first transconductance and the second transconductance, wherein the transistor pair and the cross-coupled transistor pair are mutually coupled through a plurality of inductors, the transistor pair is coupled to the first current source, the first transconductance is changed in response to a current value of the first current source, the cross-coupled transistor pair is coupled to the second current source, and the second transconductance is changed in response to a current value of the second current source.
 15. The method for generating the clock signal as claimed in claim 14, wherein the oscillator is a crystal-free oscillator.
 16. The method for generating the clock signal as claimed in claim 14, wherein the oscillator is a voltage-controlled oscillator configured to generate the oscillation signal according to an input voltage, wherein the step of determining the first transconductance of the transistor pair according to the first current source, and determining the second transconductance of the cross-coupled transistor pair according to the second current source comprises: adjusting the current value of at least one of the first current source and the second current source according to the input voltage, so as to correspondingly change at least one of the first transconductance of the transistor pair and the second transconductance of the cross-coupled transistor pair.
 17. The method for generating the clock signal as claimed in claim 16, wherein the step of adjusting the current value of at least one of the first current source and the second current source according to the input voltage, so as to correspondingly change at least one of the first transconductance of the transistor pair and the second transconductance of the cross-coupled transistor pair comprises at least one of two following steps: adjusting the current value of the first current source, so as to change the first transconductance of the transistor pair; and adjusting the current value of the second current source, so as to change the second transconductance of the cross-coupled transistor pair.
 18. The method for generating the clock signal as claimed in claim 14, wherein the inductors comprise a first inductor, a second inductor, a third inductor and a fourth inductor, the first inductor and the third inductor form a first mutual inductor, and the second inductor and the fourth inductor form a second mutual inductor, and the first mutual inductor and the second mutual inductor are physically isolated.
 19. The method for generating the clock signal as claimed in claim 14, further comprising: generating a temperature parameter, and using at least one of a third current source and a fourth current source to adjust the current value of at least one of the first current source and the second current source according to the temperature parameter.
 20. The method for generating the clock signal as claimed in claim 19, wherein the third current source is selected from one of a current source proportional to absolute temperature (PTAT) and a current source complementary to absolute temperature (CTAT), and the fourth current source is selected from another one of the current source proportional to absolute temperature and the current source complementary to absolute temperature.
 21. The method for generating the clock signal as claimed in claim 14, further comprising: receiving a compensation signal, and outputting a compensation current according to the compensation signal, so as to adjust the current value of at least one of the first current source and the second current source. 